Our principal objective is to determine the three-dimensional structures of immunoglobulins from patients with multiple myeloma and/or amyloidosis. We are currently emphasizing the serum IgG1 protein and the urinary Bence Jones dimer from the patient Mcg. A low resolution (6.5-A) electron density map for the Mcg IgG1 protein has been interpreted by a combination of computer graphics and a computer search program provided by Navia, Silverton and Davies at N.I.H. The Mcg Bence Jones dimer crystallizes in a trigonal form in ammonium sulfate and in an orthorhombic form in water. We previously solved the structure of the original form to 2.3-Angstrom resolution and built an atomic model. An electron density map for he orthorhombic form has been interpreted at 5-Angstrom resolution with the aid of a computer graphics system, and the analysis is proceeding to 2.0-Angstrom resolution with phase extension and refinement techniques. It is already clear that there are marked differences in the crystal structures of the Mcg Bence Jones dimer in the two solvent systems. These results emphasize that one amino acid sequence in an immunoglobulin gene product can give rise to multiple 3-D conformations. Crystals of other Bence Jones proteins, hybrids of light chains, and antigen-binding fragments have been obtained and examined by X-ray diffraction methods.